Magnetic resonance imaging (MRI) is a method for obtaining an image of a tissue or structure or the like in the living body using nuclear magnetic resonance in a magnetic field.
The signal strength of magnetic resonance (MR) depends on the longitudinal relaxation time (T1), transverse relaxation time (T2) and the like. Consequently, the contrast of the resulting image may be increased by controlling T1 or T2.
For example, since a gadolinium ion (Gd3+) exhibits paramagnetic properties and reduces T1 and T2, it is expected to have a significant effect as an MRI contrast agent. However, a free gadolinium ion has biological toxicity. Accordingly, a complex (hereinafter referred to as a “gadolinium complex”), which is stabilized by coordinating an organic ligand to a gadolinium ion, has been used as a contrast agent, and many proposals have been made regarding it (refer to U.S. Pat. No. 5,695,739, U.S. Pat. No. 5,798,092, U.S. Pat. No. 6,039,931, U.S. Pat. No. 6,149,890 and U.S. Pat. No. 6,177,562).
The T1-reducing effect of Gd3+ is considered to depend on the coordination of a free water molecule to Gd3+. On the other hand, a gadolinium complex, which is generally used as an MRI contrast agent, forms a chelate complex by the coordination of 7 or 8 coordinating functional groups (N or COOH) in a ligand to Gd3+. If Gd3+ is a metal ion in a nine-coordination geometry, it is more likely that the T1-reducing ability intrinsic to Gd3+ is not sufficiently derived from such a general gadolinium complex. This point is the same as in the case of a complex using a metal ion other than Gd3+.